Recording sheet curl correcting mechanism and recording sheet curl correcting method

ABSTRACT

A recording sheet curl correcting mechanism corrects curl produced in a recording sheet and for use in an image forming apparatus in that heat of a heat source is transferred to the recording sheet when images are printed while the recording sheet being conveyed. The correcting mechanism includes a curl correcting unit capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, and the curl correcting unit is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese Patent Application JP 2006-015580 filed in the Japanese Patent Office on Jan. 24, 2006, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a recording sheet curl correcting mechanism for correcting the curl produced in a recording sheet, an image forming apparatus having such a recording sheet curl correcting mechanism, and a recording sheet curl correcting method, and in particular it relates to a recording sheet curl correcting mechanism, an image forming apparatus, and a recording sheet curl correcting method that are capable of efficiently correcting the curl and stabilizing the state of the recording sheet during discharging.

2. Description of the Related Art

An image forming apparatus, such as a thermal printer, has been known that includes an apparatus of the type using cut paper as a recording sheet and that of the type using roll paper. Recently, with digital cameras becoming popular, the roll paper has been widely used, which is advantageous in cost for printing high quality images. The roll paper is made of a long recording sheet wound around a paper core cylindrically, and the necessary recording sheet is rewound and fed in a printing occasion so that the recording sheet is cut into predetermined sizes for discharging after completion of the printing.

FIG. 8 is a perspective view of an internal structure of such a conventional thermal printer 100.

As shown in FIG. 8, within the thermal printer 100, a thermal head 110 is arranged having a plurality of heating elements (heating resisters, for example). A platen roller 111 is arranged at a position opposing the thermal head 110.

Between the thermal head 110 and the platen roller 111, an ink ribbon 31 (not shown) is pinched. The ink ribbon 31 is configured so that a plurality of different color inks (yellow (Y), magenta (M), and cyan (C), for example); and a transparent laminate ink (L) are sequentially and repeatedly applied on a base film in a direction perpendicular to the winding off direction of the ink ribbon.

Furthermore, within the thermal printer 100, the roll paper 41 is held with a paper holder 42. The rewound roll paper 41 passes between conveying rollers (a capstan roller 112 and a pinch roller 113) arranged upstream a platen roller 111 in the feeding direction of the roll paper 41 and between the thermal head 110 and the platen roller 111 so as to be cut by a cutter 118, and then discharged.

FIG. 9 is a flowchart showing the printing operation of the conventional thermal printer 100 shown in FIG. 8.

As shown in FIG. 9, after starting operation, the roll paper 41 is fed at first Step S101; upon receiving the printing instruction at next Step S102, the ink ribbon 31 (Yellow Y) is printed at next Step S103 and at Step S104.

That is, at Step S103, by counterclockwise rotating the capstan roller 112 (see FIG. 8), the leading end of the roll paper 41 is fed to the point of the thermal head 110 (see FIG. 8) so that the print starting point in the middle of the way of the roll paper 41 opposes the heating element of the thermal head 110. Thereafter, the thermal head 110, which has been raised, is lowered so as to pinch the ink ribbon 31 and the roll paper 41 between the heating element of the thermal head 110 and the platen roller 111.

At sequential Step S104, by clockwise rotating the capstan roller 112 in reverse, the roll paper 41 is returned to its leading end in the reverse direction to that at Step S103. Also, by clockwise rotating the winder reel 33 (see FIG. 8), the ink ribbon 31 is wound at the same speed and in the same direction as those of the roll paper 41. Simultaneously, the heating element arranged in the thermal head 110 is selectively driven by turning on electricity, so that the color ink (Y) is printed on the roll paper 41 ranging from the print starting point to the completion point of the printing by applying thermal energy to the ink ribbon 31.

Accordingly, in the conventional thermal printer 100 shown in FIG. 8, at Step S103, the roll paper 41 is fed to the print starting point; at Step S104, the roll paper 41 is returned to be printed, so that the roll paper 41 makes one reciprocation with feeding and returning for printing the ink ribbon 31(yellow Y) thereon.

Similarly, at Step S105, the roll paper 41 is fed to the print starting point; at Step S106, the roll paper 41 is returned to print the ink ribbon 31(magenta M) thereon, so that the roll paper 41 makes two reciprocations up to here including printing yellow (Y). Furthermore, at Step S107, the roll paper 41 is fed to the print starting point; at Step S108, the roll paper 41 is returned to print the ink ribbon 31(cyan C) thereon, so that the roll paper 41 makes a third reciprocation at this time. Furthermore, at Step S109, the roll paper 41 is fed to the print starting point; at Step S110, the roll paper 41 is returned to transfer the ink ribbon 31 (laminate L) thereon, so that the roll paper 41 makes a fourth reciprocation at this time.

In such a manner, in the conventional thermal printer 100 shown in FIG. 8, the color inks (Y, M, and C) are printed and then, the laminate ink (L) is transferred, so that the roll paper 41 must make four reciprocations. The transferring of the laminate ink (L), as mentioned above, is performed while the roll paper 41 being returned, so that for cutting the roll paper 41 after the completion of printing, the print finishing point of the roll paper 41 has to be fed to the cutting position at Step S111. Then, the roll paper 41 has made four and half reciprocations up to this time, and thereafter, the roll paper 41 is cut and discharged at final Step S112 to complete printing.

As a result, in the conventional thermal printer 100, from the print starting to the completion, the roll paper 41 makes four and half reciprocations including the feeding the roll paper 41 after the print completion. Since the roll paper 41 is cylindrically wound in advance, the rewound roll paper 41 has curl generated thereon. The curl cannot be naturally corrected even the roll paper 41 makes four and half reciprocations, and as the roll paper 41 is being consumed, i.e., from the roll tongue of the roll paper 41 toward the core, the curl becomes marked.

Therefore, if the rewound roll paper 41 is printed and cut, and then is discharged in its natural state, as shown in FIG. 8, the roll paper 41 has the curl convex on the recording surface side, resulting in largely deteriorated print quality such as deformed printed images and a disturbed building up state. Moreover, this state is gradually deteriorated and the curl cannot be eliminated as it is.

Then, various techniques for correcting the curl of the roll paper 41 have been disclosed. That is, a curl correcting method is known in that along the conveying path of the rewound roll paper 41, part of the roll paper 41 is mechanically pressurized so as to simply bend the roll paper 41 in a direction opposite to that of the curl. In this curl correcting method, curl correcting means, such as a de-curl roller, is made abut the roll paper 41, and the de-curl roller can be moved or evacuated.

As another technique, a method is known in that the curl is corrected by heating one side of the roll paper 41 so as to use the bifacial difference in thermal contraction. In this curl correcting method, the roll paper 41 is heated by heating means and the heating means includes a heating body such as a plate heater and a halogen heater.

For example, in Japanese Patent Publication No. 3491524, a thermal printer is disclosed in that the printed roll paper is made abut a curl correcting member so as to change the discharging direction. According to this technique, the curl correcting effect can be adjusted using a detection lever for detecting the outer diameter of the roll paper, so that the increasing curl with decreasing outer diameter of the roll paper can be corrected.

In Japanese Unexamined Patent Application Publication No. 2002-128360, a curl correcting method is disclosed that has a curl correcting head for applying damage to roll paper by pressing it into contact. This curl correcting head reciprocates in a direction perpendicular to the conveying direction of the roll paper so as to bear down on the rear side of the roll paper.

Furthermore, in Japanese Unexamined Patent Application Publication No. 2004-223715, a thermal printer is disclosed in that the residual amount of the roll paper is detected and when the residual amount is reduced to small, a feed roller is vertically moved up and down to apply a tension to the roll paper so that the roll paper becomes curved in the opposite direction.

Moreover, in Japanese Unexamined Patent Application Publication No. 2000-318194, a thermal printer is disclosed that has a curl correcting thermal head for heating the rear side of the roll paper so as to correct the curvature of the roll paper by incurvating the roll paper in the opposite direction.

Also, in Japanese Unexamined Patent Application Publication No. 60-2559, a curl correcting device is disclosed that has curl correcting means for bending the roll paper in a direction opposite to that of curl and heating means (a heater) for heating the bent roll paper, which are arranged along the conveying path.

SUMMARY OF THE INVENTION

Any of the techniques disclosed in from Japanese Patent Publication No. 3491524 to Japanese Unexamined Patent Application Publication No. 60-2559 additionally has a mechanical curl correcting means or heating means for correcting the curl, which are arranged along the conveying path of the roll paper.

However, recently, the roll paper has been diversified in material, thickness, and standard size, so that the degree range of the curl produced in the roll paper is widened in accordance with the diversified roll paper. Hence, the correction range of the curl is demanded to be widened in correspondence to the degree of the curl. If the mechanical curl correcting means is used for this demand, the movement of the curl correcting means has to be increased, resulting in the increase in device size and cost of parts and manufacturing. If the curl is corrected by providing the heating means, the device is increased in size due to the additional heating means and the enhanced heating means, resulting in the increase in cost of parts and manufacturing. Furthermore, the mechanical and thermal damage applied to the roll paper is increased, so that quality problems, such as durability, arise.

Accordingly, it is desirable to efficiently correct curl produced on a recording sheet, such as roll paper, with a compact mechanism as well as to suppress quality deterioration of the recording sheet with the corrected curl.

According to an embodiment of the present invention, there is provided a recording sheet curl correcting mechanism for correcting curl produced in a recording sheet and for use in an image forming apparatus in that heat of a heat source is transferred to the recording sheet when images are printed while the recording sheet being conveyed. The correcting mechanism includes a curl correcting unit capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, and the curl correcting unit is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet.

In the recording sheet curl correcting mechanism according to the embodiment of the present invention, there is provided the curl correcting unit capable of bending the recording sheet in a direction opposite to that of curl of the recording sheet. Hence, curl can be corrected by bending the recording sheet in the direction opposite to that of the curl by the curl correcting unit. The curl correcting unit is arranged downstream the heat source in the feeding direction of the recording sheet ranging from the feed side toward the discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet. Therefore, the same effect as that of a case where curl is corrected by heating the recording sheet can be obtained, so that the curl of the recording sheet can be securely corrected while mechanical and thermal damage applied to the recording sheet being suppressed to the minimum.

According to the embodiment of the present invention, during printing, the recording sheet is bent in the direction opposite to that of the curl produced in the recording sheet using after heat remaining in the recording sheet, so that the curl can be efficiently corrected with a compact mechanism without an additional heating unit. The state of the recording sheet during discharging can also be stabilized, suppressing quality deterioration of the curl-corrected recording sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a thermal printer according to an embodiment;

FIG. 2 is a perspective view of the thermal printer according to the embodiment shown in FIG. 1, showing a state of a door of the thermal printer being opened and of an ink cassette being taken out;

FIG. 3 is a plan view of an ink ribbon used in the thermal printer according to the embodiment shown in FIG. 1;

FIG. 4 is a schematic side view of the internal structure of the thermal printer according to the embodiment shown in FIG. 1;

FIG. 5 is a flowchart showing the printing operation of the thermal printer according to the embodiment shown in FIG. 1;

FIG. 6 is a side view showing the vicinity of the thermal head shown in FIG. 4 and a state at Steps S3, S5, and S7 in the flowchart of FIG. 5;

FIG. 7 is a side view showing the vicinity of the thermal head shown in FIG. 4 and a state at Step S10 in the flowchart of FIG. 5;

FIG. 8 is a perspective view of the internal structure of a conventional thermal printer; and

FIG. 9 is a flowchart showing the printing operation of the conventional thermal printer shown in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described below with reference to the drawings. In the embodiment below, as an image forming apparatus according to the embodiment, a thermal printer 1 is exemplified. A recording sheet curl correcting mechanism according to the embodiment is to be mounted on the thermal printer 1.

FIG. 1 is a perspective view of the thermal printer 1 according to the embodiment.

As shown in FIG. 1, the thermal printer 1 includes a casing 2 and a door 3 attached on the front face of the casing 2. Below the door 3, an outlet 8 is formed, to which a discharge tray 9 is attached.

On the front side of the casing 2, a power switch 4 is provided. On the rear side of the casing 2, an interface 5 composed of a plurality of connectors for external connection is arranged. Image information and so forth can be obtained from a computer through the interface 5. The image information may also be read by connecting various recording media, such as a magnetic disk, an optical disk, and a memory card, to the interface 5, not from the computer.

On the other hand, on the door 3, an operation panel 6 is provided for setting the printing in the thermal printer 1. The information obtained from the interface 5 or inputted or selected by the operation panel 6 and other various messages are displayed on a liquid crystal panel 7.

FIG. 2 is a perspective view of the thermal printer 1 according to the embodiment shown in FIG. 1 in a state in that the door 3 is opened and an ink cassette 30 is taken out; the outer shape of the ink cassette 30 is shown with dotted lines for clearly showing its internal structure.

As shown in FIG. 2, a thermal head 10 is arranged on the rear side of the door 3. On the other hand, within the casing 2, a platen roller 11 (corresponding to a platen according to the present invention) is arranged at a position opposing the thermal head 10 when the door 3 is closed.

Within the casing 2, above the platen roller 11, reel driving means 14 a is provided for rotating a feed reel 32 of the ink cassette 30. Furthermore, below the platen roller 11, reel driving means 14 b is provided for rotating a winding reel 33 of the ink cassette 30.

When the ink cassette 30 is mounted on the casing 2, an ink ribbon 31 is arranged so as to pass through over the platen roller 11. Furthermore, when the door 3 is closed, the ink ribbon 31 is positioned between the thermal head 10 and the platen roller 11. When the reel driving means 14 b is driven in this state, the ink ribbon 31 is wound around the winding reel 33.

FIG. 3 is a plan view of the ink ribbon 31 used in the thermal printer 1 according to the embodiment shown in FIG. 1.

As shown in FIG. 3, the ink ribbon 31 is looped around between the feed reel 32 and the winder reel 33 arranged within the ink cassette 30 (see FIG. 2). Color inks (sublimation dyes) yellow (Y), magenta (M), and cyan (C); and a transparent laminate ink (L) are sequentially and repeatedly applied on a zonal plastic resin film in a direction perpendicular to the winding off direction of the ink ribbon 31. The laminate ink (L) protects printed images from a UV ray so as to improve the light stability by covering the printed images formed by the color inks (Y, M, and C).

FIG. 4 is a schematic side view of the internal structure of the thermal printer 1 according to the embodiment shown in FIG. 1.

Within the casing 2 shown in FIG. 1, a roll paper 41 (a recording sheet according to the present invention and wound around cylindrically) is held with a paper holder 42 as shown in FIG. 4. Then, the roll paper 41, wound off by a feed roller pair 14 on demand, passes between a capstan roller (drive roller) 12 and a pinch roller (follower roller) 13, which are arranged upstream the thermal head 10 and the platen roller 11 in the feeding direction of the roll paper 41 (to the left in FIG. 4), and then it passes between the thermal head 10 and the platen roller 11, so as to be cut by a cutter 18 and discharged.

The capstan roller 12 and the pinch roller 13 herein are equivalent to conveying rollers according to the present invention. By rotating the capstan roller 12 in a forward/reversal direction according to need, the roll paper 41 can be conveyed in a feeding/returning direction. In the thermal printer 1 according to the embodiment, the platen roller 11 is also rotated so as to convey the roll paper 41 in the feeding direction.

The conveying the roll paper 41 in the feeding direction is made by cooperation of the capstan roller 12 with the capstan roller 12. At this time, the driving mechanism of the platen roller 11 is provided with a torque limiter attached thereto and the peripheral speed of the platen roller 11 during free rotation is set faster than that of the capstan roller 12, so that when the roll paper 41 is conveyed in the feeding direction, a tension is always applied in the roll paper 41 between the capstan roller 12 and the platen roller 11. Hence, the roll paper 41 is not slackened so as to prevent conveying troubles such as buckling, securing printing operation.

Between the conveying rollers (the capstan roller 12 and the pinch roller 13) and the platen roller 11, a pair of conveying guides 15 are arranged for restricting front-face (recording surface) and rear-face positions of the roll paper 41. Each of the conveying guides 15 is partly formed in parallel with a tangent line connecting between the capstan roller 12 and the platen roller 11 so as to guide the conveyed roll paper 41 onto the platen roller 11. In addition to the cooperation of the capstan roller 12 with the capstan roller 12, owing to the positional restriction by the conveying guides 15, the roll paper 41 can be securely fed between the thermal head 10 and the platen roller 11 without rampaging due to changes in load or without buckling due to collision of the paper leading end to the platen roller 11 during paper feeding.

On the other hand, the ink ribbon 31 is unwound out of the feed reel 32 in accordance with color-converted gradation data and then is rewound by the winder reel 33 through two guide rollers 19. Hence, the ink ribbon 31 and the roll paper 41 are pinched between the thermal head 10 and the platen roller 11 in a descended state of the thermal head 10, so that the thermal head 10 presses the front face (the recording surface) of the roll paper 41 with the ink ribbon 31 therebetween.

The thermal head 10 is provided with a plurality of heating resistors (corresponding to heating elements according to the present invention) arranged in line in the width direction (the line direction) of the roll paper 41. When the roll paper 41 and the ink ribbon 31 are conveyed in the feeding direction, by selectively turning on the heating resistor of the thermal head 10 with electricity, the color inks Y, M, and C applied on the ink ribbon 31 are transferred on the recording surface of the roll paper 41 using thermal energy generated by the turning on of electricity. Similarly, the laminate ink (L) of the ink ribbon 31 is covered on the printed images.

The ink ribbon 31, from which the ink has been transferred on the roll paper 41 when the ink ribbon 31 is overlapped with the roll paper 41 in between the thermal head 10 and the platen roller 11, is peeled off the roll paper 41 by a ribbon separating member 16 arranged downstream the thermal head 10 and the platen roller 11 in the feeding direction of the roll paper 41 and then is rewound around the winder reel 33.

The ribbon separating member 16 is a guide member for changing the conveying direction of the ink ribbon 31 at a steep angle. That is, upon completion of the ink transfer, the ink ribbon 31 becomes strongly stuck on the roll paper 41 due to the pressure and the heat of the thermal head 10. Then, the ink ribbon 31 is peeled off the roll paper 41 by making the edge of the blade-like ribbon separating member 16 abut the ink ribbon 31 so as to separate the ink ribbon 31 at a predetermined angle to the feeding direction of the roll paper 41.

On the other hand, the roll paper 41 is bent in a direction opposite to that of curl produced on the roll paper 41 (upward in FIG. 4) by a de-curl roller 17 (corresponding to curl correcting means according to the present invention) for correcting the curl. Thereafter, the roll paper 41 is cut into predetermined sizes by the cutter 18 and discharged.

FIG. 5 is a flowchart of the printing operation of the thermal printer 1 according to the embodiment shown in FIG. 1.

FIGS. 6 and 7 are side views showing the vicinity of the thermal head 10 shown in FIG. 4; FIG. 6 shows the state at Step S3, Step S5, and Step S7 in the flowchart of FIG. 5; and FIG. 7 shows the state at Step S10 in the flowchart.

As shown in FIG. 5, after print starting, the roll paper 41 (see FIG. 4) is rewound and fed at first Step S1 by rotating the roll paper 41; upon receiving the printing instruction at next Step S2, the ink ribbon 31 (Yellow Y) (see FIG. 3) is printed at next Step S3. Normally, the roll paper 41 has been already fed so as to wait over the platen roller 11 (see FIG. 4), so that the printing is executed at Step S3 by the printing instruction at Step S2 without passing through Step S1.

At Step S3, by counterclockwise rotating the capstan roller 12 shown in FIG. 6, the roll paper 41 pinched to the pinch roller 13 is fed toward the platen roller 11 via between the conveying guides 15. Also, the platen roller 11 is counterclockwise rotated. At this time, when a sensor (not shown) detects the leading end (the starting point of printing) of the roll paper 41, the number of steps of a motor (not shown) for driving the platen roller 11 is counted. When the counted number reaches a predetermined number, the heating resistor of the thermal head 10 is driven by turning on electricity. Then, the thermal head 10, which has been raised, is lowered so as to pinch the ink ribbon 31 and the roll paper 41 between the heating resistor of the thermal head 10 and the platen roller 11.

Then, from this state, the roll paper 41 is fed in a feeding direction of the roll paper 41 from the feeding side toward the discharging side (to the left in FIG. 6) by counterclockwise rotating the platen roller 11 and the capstan roller 12 with the cooperation of the capstan roller 12 with the capstan roller 12. Thereby, a tension is applied on the roll paper 41, so that the roll paper 41 is smoothly conveyed in the feeding direction from the print starting point to the print completion point while being pinched between the thermal head 10 and the platen roller 11 and being positionally restricted by the conveying guides 15.

While the roll paper 41 being conveyed, the ink ribbon 31 is also rewound at the same direction and speed as those of the roll paper 41 by counterclockwise rotating the winder reel 33 (see FIG. 4). Then, the color ink (Yellow Y) of the ink ribbon 31 is transferred on the roll paper 41 sequentially from the end portion (the print starting point) due to the thermal energy applied to the ink ribbon 31 from the heating resistor of the thermal head 10.

At this time, as described above, the heating resistor is driven before the thermal head 10 pinches the roll paper 41, and the end portion (the print starting point) of the roll paper 41 at this time is immediately adjacent to a portion pinched between the thermal head 10 and the platen roller 11, so that the transferring is executed slightly before the roll paper 41 is pinched. Hence, in the thermal printer 1 according to the embodiment, the color ink (Yellow Y) can be printed on the roll paper 41 from the print starting point to the print completion point absolutely without forming a blank space in the leading end portion of the roll paper 41. While the color inks (Y, M. and C) are printed, the de-curl roller 17 is located at an evacuated position as shown in FIG. 6 so as not to bend the roll paper 41.

After the color ink (Yellow Y) is printed in such a manner, at next Step S4, the preparation is made for printing the color ink (Magenta M) at Step S5. That is, since in the color printing, the transferring of the color ink is executed every color, after the thermal head 10, which has been lowered, is raised every change of color to be transferred so as to cancel the pressurizing the roll paper 41 by the thermal head 10, the roll paper 41 is returned to the print starting point by clockwise rotating the capstan roller 12 in reverse. In addition, the platen roller 11 is provided with a one-way clutch (not shown) assembled between the rotational shaft and the motor, and the platen roller 11 rotates following the returning of the roll paper 41, so that additional loads cannot be applied to the capstan roller 12 and the roll paper 41.

Accordingly, in the thermal printer 1 according to the embodiment, at Step S3, the printing is performed up to the print completion point while the roll paper 41 being fed from the feeding side toward the discharging side (to the left in FIG. 6), and then, at Step S4, the roll paper 41 is returned to the print starting point, so that the roll paper 41 makes one reciprocation with feeding and returning the roll paper 41 for printing the color ink (Yellow Y) of the ink ribbon 31.

At next Step S5, in the same manner as at Step S3, the color ink (Magenta M) is printed while the roll paper 41 being fed from the print starting point to the print completion point. At Step S6, in the same manner as at Step S4, the roll paper 41 is returned to the print starting point. Therefore, the color ink (Magenta M) is printed at a second reciprocation. At Step S7, the color ink (Cyan C) is printed while the roll paper 41 being fed to the print completion point, and at Step S8, the roll paper 41 is returned to the print starting point, so that the color ink (Cyan C) is printed at a third reciprocation of the roll paper 41.

Furthermore, at Step S9 and Step S11, the laminate ink (L) of the ink ribbon 31 is transferred while the roll paper 41 being fed to the print completion point. This laminate ink (L) is finally transferred, so that the returning the roll paper 41 required for the transferring the next ink is unnecessary. That is, after the roll paper 41 is fed to the print completion point in the feeding direction for transferring the laminate ink (L), by continuously conveying the roll paper 41 in the feeding direction, the roll paper 41 can be cut at the print completion point for discharging without returning the roll paper 41.

In such a manner, in the thermal printer 1 according to the embodiment, although the roll paper 41 must make three reciprocations for printing the color inks (Y, M, and C), when the final laminate ink (L) is transferred and the roll paper 41 is discharged, the returning the roll paper 41 is not required. Therefore, the roll paper 41 makes three and half reciprocations from the print starting to the completion of the printing, so that the printing time in the thermal printer 1 according to the embodiment can be reduced by one reciprocation in comparison to the conventional thermal printer 100 (see FIGS. 8 and 9) which requires to make four and half reciprocations.

Also, in the thermal printer 1 according to the embodiment, immediately after the transferring the laminate ink (L) at Step S9 (print starting), at Step S10, the de-curl roller 17 abuts the rear side of the roll paper 41 as shown in FIG. 7 so as to correct the curl produced on the roll paper 41 by upward bending the roll paper 41.

The de-curl roller 17 is a metallic roller that is rotated by abutting the conveyed roll paper 41 to follow the roll paper 41, and its rotating axis is supported by a support arm 20. The support arm 20 is swingable about the rotation shaft of the platen roller 11, to which one end of the de-curl roller 17 is fitted, and the de-curl roller 17 is attached to the other end as a free end. Hence, the de-curl roller 17 swings about the rotation shaft of the platen roller 11 to be able to abut to or separate from the roll paper 41.

In order to make the de-curl roller 17, which has been located at the evacuating position shown in FIG. 6, abut the roll paper 41, a cam 22 is counterclockwise rotated from the state of FIG. 6 to the sate of FIG. 7 so as to raise a cam follower 21 of the support arm 20. Then, the de-curl roller 17 is raised to abut the rear side of the roll paper 41. Up to Step S9, the cam 22 is separated from the cam follower 21 so that the de-curl roller 17 is located at the evacuating position and does not abut the roll paper 41; at Step S10, when the end portion of the roll paper 41 passes through the ribbon separating member 16, the cam 22 is rotated so as to make the de-curl roller 17 abut the rear side of the roll paper 41, as shown in FIG. 7, for correcting the curl of the roll paper 41.

That is, the roll paper 41, as shown in FIG. 4, is made of the recording sheet wound around with its recording surface facing the outside, and it develops a curl convex on the front face side (the recording surface side). Hence, when the roll paper 41 having the laminate ink (L) transferred on the recording surface at Step S9 is discharged as it is, the curl convex on the front face side directly remains. Then, at Step S10, for bending the roll paper 41 in a direction opposite to that of the curl produced on the roll paper 41 (the concave direction on the front face side and upward in FIG. 7), the de-curl roller 17 is abutted on the rear side of the roll paper 41.

Moreover, the de-curl roller 17 is arranged downstream the platen roller 11 in the feeding direction of the roll paper 41 (to the left in FIG. 7) as well as immediately adjacent to the ribbon separating member 16. In other words, the de-curl roller 17 is arranged close to the thermal head 10 using a space downstream the platen roller 11 unoccupied by arranging the capstan roller 12 upstream the platen roller 11. Accordingly, at the abutting position of the de-curl roller 17 to the roll paper 41, the after heat due to the thermal head 10 during the transferring the laminate ink (L) remains in the roll paper 41.

When the roll paper 41 is bent in a direction opposite to that of the curl (upward in FIG. 7) by making the de-curl roller 17 abut the rear face side of the roll paper 41 in a state of the after heat remaining in the roll paper 41, the roll paper 41 is deformed before the completion of the thermal contraction of materials constituting the roll paper 41 and the moisture loss, so that the same effect as the case where curl is corrected by heating paper can be obtained. Hence, the curl of the roll paper 41 can be securely corrected. Also, the curl can be efficiently corrected without arranging heating means such as a heater.

The degree of the curl produced in the roll paper 41 varies with the environmental conditions during printing (temperature, humidity, etc.), the material of the roll paper 41, the conveying speed of the roll paper 41, and the outer diameter of the roll paper 41. In the thermal printer 1 according to the embodiment, the bending degree of the roll paper 41 is changed in accordance with the curl degree produced on the roll paper 41 so as to adjust the curl correction. Specifically, as shown in FIG. 7, the surface of the cam 22 is inclined, so that by adjusting the rotational angle on the basis of the environmental conditions during printing, the material of the roll paper 41, the conveying speed, and the roll diameter, the lift degree of the cam follower 21 by the cam 22 can be continuously changed.

For example, the shape and degree of the curl of the roll paper 41 gradually change from the roll tongue toward the core, and the degree of the curl becomes marked toward the roll core. By bending the roll paper 41 so much for that with the de-curl roller 17, the curl can be uniformly corrected so as to stabilize the state of the roll paper 41 during discharging. The curl can also be corrected by user's preference according to setting.

After print starting at Step S9, the curl is corrected in such a manner at Step S10, and the roll paper 41 is finished printing at Step S11, and then it is cut by the cutter 18 (see FIG. 4) at Step S12 for discharging. The discharged roll paper 41 has high quality due to the corrected curl, and does not scatter in the vicinity of the outlet 8 (see FIG. 1). In the thermal printer 1 according to the embodiment, during cutting the roll paper 41, the roll paper 41 is cut at positions slightly on this and the other side of the boundary between the print completion point and an unprinted portion. Accordingly, the curl of the roll paper 41 is not only corrected but also the size of sheet scraps can be reduced to about few millimeters.

At final Step S13, the cam 22 is reversely rotated from the state of FIG. 7 to that of FIG. 6 so as to lower the de-curl roller 17 for moving it to the evacuation position, so that the printing is finished in a state in that the next printing is not disturbed. In the thermal printer 1 according to the embodiment, sheet scraps with a size of few millimeters are generated; however, no scrap can also be generated.

Therefore, the thermal printer 1 according to the embodiment can combine the reduction in sheet scrap with the reduction in printing time. That is, even when the conveying rollers (the capstan roller 12 and the pinch roller 13) are arranged upstream the platen roller 11, by the cooperation of the capstan roller 12 with the platen roller 11, the printing can be performed while the roll paper 41 being conveyed in the feeding direction. Hence, the blank space, generated due to the conveying of the roll paper 41 with conveying rollers arranged downstream the platen roller 11, can be eliminated, so that the sheet scraps can be reduced to be an only slight amount for ordering the vicinity of the boundary between the print completion point and an unprinted portion. In comparison with the printing while the roll paper 41 being conveyed in the returning direction, the number of reciprocations between the print stating and the print completion can be reduced by one reciprocation, reducing the printing time.

Furthermore, in the thermal printer 1 according to the embodiment, in a state of after heat during printing remaining in the roll paper 41, the de-curl roller 17 bends the roll paper 41 in a direction opposite to that of the curl, so that the curl can be efficiently corrected with a broad adjusting range with a compact mechanism and without additional heating means and loads applied to parts cost and manufacturing cost. The printing quality such as durability can be obtained by suppressing mechanical and thermal damage applied to the roll paper 41 to the minimum.

(1) According to the embodiment, as an image forming apparatus, the thermal printer 1 is exemplified; however, other than the thermal printer 1, various image forming apparatuses, such as other system printers and copying machines, may be incorporated. That is, the recording sheet curl correcting mechanism according to the embodiment may be arranged downstream a fixing device of a copying machine for correcting curl.

(2) The roll paper 41 is used as a recording sheet according to the embodiment; alternatively, a cut sheet may also be used; that is, even in the cut sheet, the printing time can be reduced; and curl may be produced due to heat of a fixing device even in the cut sheet, and the curl can be efficiently corrected.

It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 

1. A recording sheet curl correcting mechanism for correcting curl produced in a recording sheet and for use in an image forming apparatus in that heat of a heat source is transferred to the recording sheet when images are printed while the recording sheet being conveyed, the correcting mechanism comprising curl correcting means capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, wherein the curl correcting means is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet.
 2. The mechanism according to claim 1, wherein the curl correcting means is movable so as to abut to or separate from the recording sheet.
 3. The mechanism according to claim 1, wherein the curl correcting means changes the degree of the bending the recording sheet in accordance with environmental conditions during the printing.
 4. The mechanism according to claim 1, wherein the curl correcting means changes the degree of the bending the recording sheet in accordance with the material of the recording sheet.
 5. The mechanism according to claim 1, wherein the curl correcting means changes the degree of the bending the recording sheet in accordance with the conveying speed of the recording sheet.
 6. The mechanism according to claim 1, wherein when the recording sheet is cylindrically wound roll paper, the curl correcting means changes the degree of the bending the recording sheet in accordance with the outer diameter of the roll paper.
 7. An image forming apparatus comprising: a conveying roller for conveying a recording sheet; a thermal head having a heating element; and a platen arranged to oppose the thermal head for pinching the recording sheet between the platen and the thermal head during printing images on the recording sheet, wherein the image forming apparatus prints images by driving the heating element with turning on electricity while the recording sheet pinched between the thermal head and the platen being conveyed by the rotation of the conveying roller, and the image forming apparatus includes curl correcting means capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, and wherein the curl correcting means is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet.
 8. A recording sheet curl correcting method for correcting curl produced in a recording sheet used in an image forming apparatus in that heat of a heat source is transferred to the recording sheet when images are printed while the recording sheet being conveyed, the method comprising the step of bending the recording sheet in a direction opposite of that of the curl in a state of the recording sheet being positioned downstream the heat source in a feeding direction of the recording sheet ranging from a feeding side toward a discharge side as well as in a state of after heat due to the heat source during printing remaining in the recording sheet.
 9. The method according to claim 8, further comprising the step of adjusting the correction of curl produced in the recording sheet by changing the degree of the bending the recording sheet.
 10. A recording sheet curl correcting method of a recording sheet used in an image forming apparatus that includes a conveying roller for conveying a recording sheet, a thermal head for sequentially transferring a plurality of inks on an ink ribbon onto the recording sheet by driving a heating element with turning on electricity, and a platen arranged to oppose the thermal head for pinching the ink ribbon and the recording sheet between the platen and the thermal head during the transferring each ink, the image forming apparatus being for printing images by sequentially transferring each ink while the recording sheet pinched between the thermal head and the platen being conveyed by the rotation of the conveying roller, the method comprising the step of bending the recording sheet in a direction opposite of that of curl after the final ink is transferred onto the recording sheet from the ink ribbon and in a state in that the recording sheet is positioned downstream a heat source in a feeding direction of the recording sheet ranging from a feeding side toward a discharge side as well as in a state of after heat due to the heat source during printing remaining in the recording sheet.
 11. A recording sheet curl correcting mechanism for correcting curl produced in a recording sheet and for use in an image forming apparatus in that heat of a heat source is transferred to the recording sheet when images are printed while the recording sheet being conveyed, the correcting mechanism comprising a curl correcting unit capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, wherein the curl correcting unit is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet.
 12. An image forming apparatus comprising: a conveying roller for conveying a recording sheet; a thermal head having a heating element; and a platen arranged to oppose the thermal head for pinching the recording sheet between the platen and the thermal head during printing images on the recording sheet, wherein the image forming apparatus prints images by driving the heating element with turning on electricity while the recording sheet pinched between the thermal head and the platen being conveyed by the rotation of the conveying roller, and the image forming apparatus includes a curl correcting unit capable of bending the recording sheet in a direction opposite to that of curl produced on the recording sheet, and wherein the curl correcting unit is arranged downstream the heat source in a feeding direction of the recording sheet ranging from a feed side toward a discharge side as well as arranged at a position where after heat due to the heat source during printing remains in the recording sheet. 